1767 Relationships between lithofacies belts and conodont faunas, Gun River Formation (Lower Silurian), Anticosti Island, Quebec: a statistical approach Shunxin Zhang, Christopher R. Barnes, and Susanne M.L. Pohler Abstract: Discriminant analysis of 80 samples from the Western Carbonate Platform Facies and the Eastern Transitional Carbonate–Siliciclastic Platform Facies of the Gun River Formation (Lower Silurian) on Anticosti Island, Quebec, indicates the degree to which different conodont communities were related to particular lithofacies. The statistical analysis reveals that during most of the Gun River Formation deposition, the conodont distributional pattern of the Western Carbonate Platform Facies and the Eastern Transitional Carbonate–Siliciclastic Platform Facies of the formation remained stable, and that the boundary of the two facies oscillated eastward and westward. The analysis indicates quantitatively that Icriodella deflecta had a nearshore environmental preference, whereas Rexroadus kentuckyensis tended to reside in offshore environments. These two species play the most important role in differentiating conodont communities and determining the relationship between conodont communities and lithofacies among all of the 22 species known from the Gun River Formation. Résumé : Une analyse discriminante de 80 échantillons du faciès occidental de la plate-forme carbonatée et du faciès oriental de la plate-forme de transition carbonate–siliclastique de la Formation de Gun River (Silurien inférieur) sur l’île d’Anticosti, au Québec, indique le degré de relation entre les diverses communautés de conodontes et des lithofaciès particuliers. L’analyse statistique révèle que, lors de la déposition de la plus grande part de la Formation de Gun River, le patron de distribution des conodontes du faciès occidental de la plate-forme carbonatée et celui du faciès oriental de la plate-forme de transition carbonate–siliclastique de la formation sont demeurés stables et que la limite entre les deux faciès oscillait vers l’est et vers l’ouest. Les résultats quantitatifs de l’analyse indiquent que Icriodella deflecta préférait les environnements littoraux alors que Rexroadus kentuckyensis avait plus tendance à rester dans des environnements au large. Ces deux espèces sont les plus significatives pour différencier les communautés de conodontes et déterminer la relation entre les communautés de conodontes et les lithofaciès parmi toutes les 22 espèces connues dans la Formation de Gun River. [Traduit par la Rédaction] Zhang et al. 1782 Introduction distribution through biostratigraphic studies, attention turned to paleoecologic studies. Earlier work is represented by two Conodonts are common marine phosphatic microfossils of contrasting models of conodont paleoecology, i.e., the Late Cambrian – Triassic age (Sweet 1988). They have depth-stratified pelagic habit (Seddon and Sweet 1971) and demonstrated exceptional value as biostratigraphical tools the nektobenthic habit with a minority of truly pelagic forms and as geochemical and thermal maturation indicators. Much (Barnes and Fåhraeus 1975; Fåhraeus and Barnes 1975). The of the earlier work on conodonts had a taxonomic and latter research outlined the complex patterns of conodont biostratigraphic emphasis. The initial surge of work on the paleoecology. The close correlation of conodont distribution stratigraphic distribution of diagnostic conodonts was with various sedimentologic criteria was recognized, for summarized by Sweet and Bergström (1974) and Higgins example, in Late Ordovician and Early Silurian conodont and Austin (1985). As more data accumulated on conodont faunas from Hudson Bay by Le Fèvre et al. (1976) and in Received 28 February 2002. Accepted 11 October 2002. Published on the NRC Research Press Web site at http://cjes.nrc.ca on 20 December 2002. Paper handled by Associate Editor B. Chatterton. S. Zhang and C.R. Barnes.1 School of Earth and Ocean Sciences, University of Victoria, P.O. Box 3055, Victoria, BC V8W 3P6, Canada. S.M.L. Pohler.2 Geological Survey of Austria, Rasumofskygasse 23, A-1031 Vienna, Austria. 1Corresponding author (e-mail: [email protected]). 2Present address: Marine Studies Programme, University of the South Pacific, Suva, Fiji. Can. J. Earth Sci. 39: 1767–1782 (2002) DOI: 10.1139/E02-087 © 2002 NRC Canada 1 1768 Can. J. Earth Sci. Vol. 39, 2002 Triassic conodonts from Utah and Nevada by Clark and trending carbonate and siliciclastic rock facies partitioning Rosser (1976). In the latter, both descriptive methods and made by Petryk (1981). quantitative analyses were adopted, based on known ecological Further observations on stratigraphy and sedimentology factors of associated fossils and treated conodont faunas as (Long 1997) revealed that siliciclastic sand and mud were variables to be manipulated with a number of mathematical transported into the eastern end of the Anticosti Basin by techniques. Carr et al. (1984) successfully used the χ2 test, sustained longshore currents that swept the coast of discriminant analysis, and factor analysis to document that Laurentia along the west coast of the Strait of Belle Isle. It the distribution of some Early Triassic conodonts was controlled appears that the siliciclastic input was related to a period of by a depth-related environmental gradient, whereas others lowered sea level. were not. New conodont data from collections of the Gun River Other studies on Silurian conodont paleoecology (Aldridge Formation from the eastern end of Anticosti Island by and Mabillard 1981; Aldridge and Jeppsson 1984; Idris 1984) S.M.L. Pohler and P. Copper are compared with earlier identified the different conodont specialists in the nearshore conodont data from the western and central part by C.R. and offshore environments based on the conodont distribution Barnes and colleagues. The Gun River Formation comprises in Norway and Britain. Analytical studies of Late Ordovician most of the coastal outcrops on the eastern end of Anticosti and early Early Silurian conodonts from Anticosti Island, Island and hence provided the best opportunity of all the Quebec (Nowlan and Barnes 1981; McCracken and Barnes stratigraphic units to examine west–east facies and conodont 1981), outlined the pattern of conodont communities and the community changes. impact of the latest Ordovician mass extinction. Zhang and All previous Anticosti Island Silurian conodont fauna and Barnes (2002b) conducted cluster analysis, using both earlier conodont paleoecology studies, however, were restricted to and new collections, to identify 11 conodont communities the western carbonate platform belt. The present study through the Llandovery sequence on Anticosti Island and documents the differences in conodont faunas between the documented that development and replacement of the different western and eastern facies during the deposition of the Gun conodont communities were related to eustatic sea level changes. River Formation (mid-Llandovery), based on new collections Zhang and Barnes (2002c) also used cluster analysis to detail from eastern Anticosti Island. the relationship between lithofacies and conodont communities through this interval. Materials and methodology Anticosti Island lies in the Gulf of St. Lawrence, Quebec (Fig. 1). In the Late Ordovician and Early Silurian it was located The Gun River Formation spans the Oulodus jeannae about 20° south of the equator on the eastern margin of Subzone of the Ozarkodina strena Zone and lower Laurentia, bordering the Iapetus Ocean. The stratigraphic Ozarkodina clavula Zone of lower Aeronian, Llandovery (Zhang sequence through this interval is continuous, undeformed, and Barnes 2002a). The formation is superbly exposed along approximately 1100 m thick, and richly fossiliferous, consisting the coast and major rivers of Anticosti Island. Both carbonate of limestone with minor shale and sandstone. The sequence and transitional carbonate–siliciclastic lithofacies are well is subdivided into the Upper Ordovician Vauréal and Ellis developed within the formation, which enables this study of Bay formations and the Lower Silurian Becscie, Merrimack, the relationships between lithofacies and conodont faunas. Gun River, Jupiter, and Chicotte formations (Fig. 1). These Based on the type section on the south-central coast, Petryk sediments were deposited in a broad, 40–100 km wide, (1981) divided the Gun River Formation into five informal storm-influenced ramp to shelf, subjected to a series of marine members: member 1, 5 m thick, is composed of graptolite- regressions and transgressions of differing magnitudes (Petryk bearing lime mudstone with lenses of grainstone and 1981; Long and Copper 1994; Copper and Long 1998). packstone with minor shale partings; member 2 consists of In general, two northwest–southeast-trending carbonate approximately 7 m of lime mudstone and calcisiltite with and siliciclastic rock facies were identified (Petryk 1981): lenses and partings of coarser carbonate and shale; member 3, the Western Carbonate Platform Facies (abbreviated as the 105 m thick, consists mainly of lime mudstone, grainstone, western facies in the following text) and the Eastern Transitional and intrarudstone; member 4, 28 m thick, shows an increase Carbonate–Siliciclastic Platform Facies (abbreviated as the in grainstone content; member 5 isa4mthick biohermal eastern facies in the following text) (Fig. 2). The western facies unit. was developed through most of the sequence and was widely In a study of the Gun River sections along